trical power production and transport—infrastructure that will soon be bolstered by efforts to augment renewable energy production whose primary byproduct is electrical power (e.g., through photovoltaic cells and wind turbines).
If electrical energy becomes the preferred form of energy, electrochemical energy storage is a natural fit. In contrast, hydrogen fuel cell technology requires an entirely new infrastructure to efficiently produce hydrogen and then transport, store, and reconvert it to electrical energy.
To put into perspective the amount of energy consumed by the transportation sector, of the total 2.85 × 1016 watt-hours (1 Quad = 2.93 × 1014 watt-hours) of energy used by the United States in 2011 27.7% (7.91 × 1015 watt-hours) went to transportation (Figure 1).1 However, due to the relatively low chemical-to-mechanical energy conversion efficiency of internal combustion engine (ICE) technology, the ratio of serviceable to rejected energy is disproportionately low compared to other energy use sectors.
If EVs can improve energy efficiency in the short term and the technology for non-fossil-fuel-based/renewable electrical power generation can be realized in the long term, the benefits to our country’s current and future sustainability are clear. Assuming the latter, the following discussions focus on electrical energy storage, specifically batteries.
CHALLENGES FOR ELECTROCHEMICAL ENERGY STORAGE AND USE IN EVS
Defining the ideal battery for EVs is complicated because of the numerous powertrain configurations involved in HEVs, PHEVs, and BEVs; for example, the capacity (kWh), power (kW), and cycle life can be considerably different for an HEV compared to a BEV (Khaligh and Li 2010). To simplify discussion, this article focuses on BEVs with battery characteristics that can power a four-seat vehicle for approximately 100 miles on a single charge, criteria favorable for widespread adoption.2Figure 2 shows the necessary performance attributes of an effective EV battery.
Vehicle range is determined by the amount of energy stored in the battery and the rate at which the energy is expended to propel the vehicle. A 23 kWh battery used to power a ~70 kW electric motor is believed to be sufficient to achieve a range of about 160 km. The mass and volume of the battery should be minimized to reduce the vehicle mass while maximizing vehicle cabin volume, respectively.
1These data and the accompanying figure are from the Lawrence Livermore National Laboratory website, https://flowcharts.llnl.gov/content/energy/energy_archive/energy_flow_2011/LLNLUSEnergy2011.png, accessed November 9, 2012.
2Whether this BEV performance standard is specifically required to significantly impact energy consumption is not yet known, but agencies and auto companies generally agree with this definition (Bruce et al. 2012; CCC 2012; Thackeray et al. 2012).